infill is the percentage of plastic that is inside the shells. in general more infill does mean more strength. however in practice that is not always the case. if your part has to resist crushing or is going to have significant forces on it a solid or near solid infill is a good idea. this can also be applicable if your part needs to be more rigid
Most cases you can print with no infill, I often do on. even an parts that are handled a lot.
Shells are the outer layer in general 3 shells is fine you may want to add a few more if you want a stronger part but don't want a large infill. I have not seen any reduction in accuracy in small parts even with increased shells. if you part has a portion that is small enough that only 2 shells will fit in that space and you have it set to 6 you still only get 2. (well with a decent slicer)
if you want a .1 mm line you cannot get that from a .4 mm nozzle. You can get a line that is .1mm thick (if calibrated right) but it will be wider then .4 as it gets flattened and squished.
printing slower will give you more accuracy. in general it doesn't really have anything to do with allowing the plastic to cool but that you have more precise motion control. however that also allows from more time to have curling. One caveat is if the print area is super small then, yes you often do need to allow for some time for the plastic on that layer to cool before printing the next layer. A cooling fan can help with this. There are times that a cooling fan is advantages. For abs that is notable in printing over hangs, and bridges or small pillars or points. This is why its nice to be able to have a separate layer cooling fan that you can control via the slicer. it is also often why if you don't and you have a abject with a small point you print multiples or print a pillar which allows the layer time to solidify.
so lets say your trying to print a stamp for making wax seals. ( note I don't know how well this would hold up due to hot wax just using as an example. it would most likely be better to use it to make a mold that you can then cast in metal)
You wont have a lot of height so but you want it be able to withstand some impact pressure without deformation. In this case a sold infill would be good. .1 layer height will also be important since you need to keep the vertical resolution.
printing speed can be varied here to fast and you lose some x,y resolution but when printing if your having troubles printing it almost always you can try printing it slower and that may allow it to work.
Hope that helps
